A so-called "bearingless" flow sensor includes a fluid passage having means for causing the fluid to swirl in a detection zone, a ball in the detection zone that is caused to orbit by the swirling fluid, and a ball detector to produce electrical pulses representing orbits of the ball. Orbital-ball flow sensors are known which may be conveniently called the "axial-flow" type, in which fluid flows along a pipe having flow-swirling vanes at the entry and exit of the ball-orbiting zone. In another form that may be conveniently called the "toroidal" type, the fluid enters a toroidal passage containing the orbital ball, and it leaves either axially or tangentially.
Various forms of detector for the orbital ball have been devised for the toroidal type of flow sensor. In U.S. Pat. No. 4,157,660 to G. C. Spacek, toroidal flow sensors are shown having a photoelectric ball detector where the passage wall must be transparent at the detector, an inductive ball detector where a coil on a core magnet responds to a ball of magnetic material, and a capacitive type in which a ball orbits between a pair of capacitive electrodes.
Ball detectors in the axial type of orbital-ball flow sensors involve special difficulties and limitations. However, my application Ser. No. 667,766, filed Nov. 2, 1984, shows photoelectric ball detectors, and my application Ser. No. 677,873, filed Dec. 4, 1984, which is a continuation-in-part of application Ser. No. 501,810, filed June 7, 1983, now abandoned shows capacitive ball detectors in axial-flow orbital-ball flow sensors.
In each instance, there are constraints militating against the use of those flow sensors in some applications. Magnetic detectors involve drag on the ball severely distorting the otherwise linear flow-versus-pulse characteristic at low flow rates. Photoelectric ball detectors obviously require material for the flow passage that transmits light to the orbital path. Capacitive ball sensors require either exposure of the capacitive electrode to the fluid or non-metallic material for the flow passage.
In one aspect, the present invention relates to improvements in orbital-ball flow sensors, as more fully discussed below.
Various forms of flow sensors such as the orbital ball type and the vaned rotor type have a flow-vs-output characteristic that is essentially linear over a range that is limited. Accuracy of a flow sensor for a large-bore pipe, e.g. a 4-inch pipe, tends to suffer at the lower end of its range, and yet such flow might be at the middle or upper limit of the linear range of a small-bore pipe, e.g. 3/4-inch pipe. Moreover, there are variations in the flow rate at different parts of a pipe's cross-section that may require particular attention for accurate measurement. A further aspect of the invention relates to providing novel wide-range flow sensors, i.e. flow sensors operable at relatively large flow rates, yet capable of accurate response at relatively low flow rates.